1. Field of the Invention
The present invention relates generally to fuel cell devices, and particularly to frames for the fuel cell devices.
2. Technical Background
The use of solid oxide fuel cells has been the subject of considerable amount of research in recent years. The typical components of a solid oxide fuel cell (SOFC) comprise a negatively-charged oxygen-ion conducting electrolyte sandwiched between two electrodes. Electrical current is generated in such cells by oxidation, at the anode, of a fuel material, for example hydrogen, which reacts with oxygen ions conducted through the electrolyte. Oxygen ions are formed by reduction of molecular oxygen at the cathode.
U.S. Patent Publication US2002/0102450 and 2001/0044041 describe solid electrolyte fuel cells which include an improved electrode-electrolyte structure. This structure comprises a solid electrolyte sheet incorporating a plurality of positive and negative electrodes, bonded to opposite sides of a thin flexible inorganic electrolyte sheet. One example illustrates that the electrodes do not form continuous layers on electrolyte sheets, but instead define multiple discrete regions or bands. These regions are electronically connected, by means of electrical conductors in contact therewith that extend through vias in electrolyte sheet. The vias are filled with electronically conductive materials (via interconnects).
U.S. Pat. No. 5,085,455 discloses thin, smooth inorganic sintered sheets. The disclosed sintered sheets have strength and flexibility to permit bending without breaking as well as excellent stability over a wide range of temperatures. Some of the disclosed compositions, such as yttriastabilized zirconia YSZ (Y2O3—ZrO2) would be useful as electrolytes for fuel cells. It is known that at sufficient temperatures (e.g., about 725° C. and above), zirconia electrolytes exhibit good ionic conductance and very low electronic conductance. U.S. Pat. No. 5,273,837 describes the use of such compositions to form thermal shock resistant solid oxide fuel cells.
U.S. Patent Publication US2001/0044043 describes solid electrolyte fuel cells utilizing substantially planar, smooth electrolyte sheet with a roughened interface surface layer. This publication discloses electrolyte sheet thickness below 45 micrometers. The ceramic electrolyte sheet is flexible at such thicknesses.
Furthermore, fuel cells endure thermal cycling and large thermal gradients, which induces thermal stresses in the electrolyte sheets. In addition, a mounted electrolyte sheet will expand at a rate that is different from the thermal expansion rate of its frame, which may cause cracking of the electrolyte sheet. A defect in an electrolyte sheet may necessitate a replacement of entire cell or electrolyte device.
It is known that substrate type solid oxide fuel cells sometimes utilize metal alloys as separators. Such configuration is described, for example, in the article entitled “Electrochemical properties of a SOFC cathode in contact with a chromium-containing alloy separator”, by Yoshido Matsuzaki and Isami Yasuda, Solid state Ionics 132 (2000) 271-278.
Solid oxide fuel cells may also be supported by a porous support structure, as disclosed for example in U.S. Pat. No. 5,486,428. Inside the porous support structure are sealed corrugated ceramic plates that form passages for either oxygen or fuel. More specifically, U.S. Pat. No. 5,486,428 discloses fuel cell modules, each having a porous substrate supporting a plurality of electrodes. An electrolyte layer is situated over these electrodes and another electrode layer is situated on the electrolyte layer. The porous support structure forms an integrated whole with these layers and is inseparable from these layers. The patent discloses that the fuel cell layers are directly bonded to the porous support structure, therefore fabrication constraints limit fuel cell configuration. For example, the cell layers are generally fired at different temperatures. Typically the anode and electrolyte are sintered at temperatures of 1400° C. or higher, whereas the cathode is ideally sintered at a temperature of 1200° C. or lower. Hence the fuel cell layers must be deposited in decreasing order of firing temperatures. However, it would be advantageous to be able to have other design configurations of the fuel cell arrays, without concern for the fabrication constraints. Furthermore, the porous support structure is relatively thick, and therefore, expensive to make. U.S. Pat. No. 6,194,095 discloses fuel cell stacks containing fuel cell arrays formed on an electrolyte impregnated porous plastic dielectric sheets with the cell to cell electrical interconnections made through the electrolyte membrane. The disclosed design utilizes air flow manifold units as well as fuel manifold units assembled between the fuel cell arrays. Having additional air and fuel manifold units and assembling them between the fuel cell arrays increases the complexity and the cost of the fuel cell stack.
U.S. Pat. No. 5,416,057 discloses a coated alternating heat exchanger device and a method of making such. The heat exchanger comprises a plurality of passages situated within a ceramic body. This patent does not disclose the use of this device in fuel cell applications.